Horizontal Directional Drilling (HDD) is a construction method alternative to open trenching that is used for installing conduits such as cables, pipes, and the like for underground utilities. In many HDD projects, a “pilot bore” is first made using a steerable drilling tool, typically consisting of a sonde housing, which may contain tracking electronics, a thread adapter and some type of drill bit. The drill bit is typically either a flat “paddle bit” or an “offset rock bit.” This combination of sonde housing, thread adapter and drill bit may be referred to as the “drill head assembly.” Typically, the thread adapter of the drill head assembly is connected to a string of drill rods. The drill rods may be 10′-5′ in length and hollow with threaded ends. During pilot bore operations, the drill rods may be added to the drill string, one at a time, as the pilot bore proceeds further. The drill string may rotate to cut the soil and drilling fluid may be pumped through the drill rod and into the drill head assembly.
Typically, high pressure drilling fluid is pumped from the front of the housing or bit, in the general direction of the bore path, to aid in cutting the soil and steering. For most HDD projects, at the termination of the pilot bore, the drill head assembly is removed and a backreamer is attached. In one or more steps, the backreamer enlarges the hole to a size adequate for the pipe or conduit being installed, and drilling fluid is pumped through the backreamer to aid in the cutting of the soil, to condition the bore hole, and to provide a medium for carrying the cuttings out of the hole. The drilling fluid is typically pumped at a lower pressure and higher flow rate during backreaming than during the pilot bore drilling step. The product pipe is pulled into the reamed hole, behind the backreamer, in the “backream and pullback” step.
For installation of smaller utilities, such as telecommunication transmission lines or small gas distribution lines, the hole created by the pilot bore drilling step may be sufficiently large for the installed product pipe, without use of a backreamer. This simplifies the installation process by eliminating the need for removing the drill head assembly and attaching the backreamer. The product pipe can be attached to the drill head assembly by means such as shackles or swivels which attach to the drill bit. This is referred to as “direct pullback.” It can be an effective method for installing smaller product pipes over shorter distances.
U.S. Publication No. 2002/0112890 and U.S. Pat. No. 8,122,979 describe some of the known drill bit designs including features designed for direct pullback. These documents are primarily directed to the attachment of the product pipe to the drill head assembly for direct pullback. However, current sonde housings and drill bits are not optimized for direct pullback. The drilling fluid is still pumped from the front of the sonde housing assembly, at high pressure and low flow rate. This is disadvantageous because the hole may be relatively dry in the backreaming direction, resulting in possible difficulties pulling back the product pipe, potentially damaging it or causing the direct pullback operation to be unsuccessful. As a result, typically, only small product pipes can successfully be installed.
In addition, if the drill head assembly rotates in a relatively dry hole, friction may cause the sonde housing to become heated, potentially damaging the tracking electronics housed inside. Also, the high pressure drilling fluid jet may impinge upon the swivel, puller or product pipe, also potentially causing damage. The flow rate of drilling fluid may also be lower than desired for installation of the product pipe. Typically the drill head assembly will not ream the hole larger than it was cut during the pilot boring operation, nor will it effectively mix the soil cuttings with the drilling fluid. This further limits the diameters and lengths of the product pipe that can be installed. It would be advantageous to be able to use direct pullback for larger or multiple product pipes.
Accordingly, what is needed is a direct pullback device and method that overcome the aforementioned disadvantages associated with the known devices.